I can answer at least half of your question.
The ability to edit posts is restricted to one half hour across most of the forums. This means that after you post you have a half hour to edit it to your heart's content.
The ability to edit was changed to this half an hour limit several years ago (It may be 15 minutes in some forums but I believe its 30 across the board). It was prompted by some members deleting or changing their posts to discredit later posts after them (this is not the full reason, I think this was merely a component of the reason. While I was around, I wasn't a moderator until a day or two ago so I cannot give you the full administrative reasoning behind the change).
As far as the ability to delete your post, that may or may not have been a side-effect of changing the edit span (since the delete option was under the edit option).
If you need a post deleted, you can contact the moderator of the forum in question and they will delete the post for you.
If you simply want the thread to be locked (meaning it still exists on the forums but no new replies can be given- this will result in it sinking to the later pages eventually since it can't be bumped) that is also an option. Generally I believe this to be more favorable than out-right deleting but either of these options can be asked of the moderating staff in question.
It is best to ask the moderator of the forum your particular post is in, but if you cannot reach them, then I would recommend branching out to other moderators (Although I'm fairly new to this game, I am one for respecting the boundaries of the moderator staff, etc.).
I hope this answers or satisfies your queries.
Have a good one,